This invention concerns a mill for removing a packer from an oil well or the like.
A packer is a device placed in the steel casing of an oil well for isolating upper and lower sections of such casing. In a single completion packer there is a central bore and surrounding structure that seals the packer inside the casing. Tubing can be connected to or through the packer for withdrawing fluids from the well. A broad variety of packers are commercially available and the structure thereof is of no significance for this invention.
Some packers are designed for release so that they can be readily removed from the casing and others are more or less permanently fixed in the casing. Even with the readily removable packers it often occurs that corrosion or the like prevents removal. Thus, it is a reasonably common procedure in work-over of oil wells to require milling of the packer to remove it from the well. Such milling destroys the packer and milling chips are pumped out of the well or are caught in downhole debris collectors. Junk that remains in the well can be removed with a magnet or can be milled by a conventional junk mill.
As milling of the packer continues a point is ordinarily reached where the remains of the packer and any tubing or the like hanging from it are freed from the casing and fall free. A grip or catcher on the milling tool used to mill the packer catches these remains so that they can be drawn upwardly and removed from the well bore.
It sometimes occurs as the remains of the packer are lifted that they become stuck in the well bore. It may also occur that the milling tool becomes worn or damaged before the packer is free. In either of these circumstances it may be desirable to remove the milling tool while leaving the remainder of the packer in the well. It is therefore desirable to provide a means for releasing the packer mill from the packer. The mill can then be withdrawn and the well reentered with the same or a different tool for completing removal of the packer.
One approach that has been used to effect such release employs J-slots. A packer mill can be released by lowering and reversing the direction of rotation. Such arrangements are not entirely satisfactory because of the multiplicity of moving parts normally required and the substantial likelihood that such mechanisms may jam. This leaves the operator in the position of fishing the remains of the packer mill as well as the packer, or milling the entire collection of junk in the well. This is obviously costly and time consuming.
Another approach has been to provide pins, screws or stops that shear when a large lifting force is applied to the packer mill. When these parts shear, the means for catching the remains of the packer is released and the packer mill can be withdrawn. This shear type release can have problems since loose parts such as the ends of pins can be produced and require removal from the well. The loose parts themselves may cause jamming. Shear pins and bolts can cause deformation of the holes in which such items are inserted and result in difficulties in reusing the packer mill. Such damage may not be readily repaired in the field, thereby introducing additional delay during work-over.
A potential problem with a release relying on a plurality of shearing elements such as pins, bolts or stops comes from asymmetry in the well or packer being removed. This can result in sequential failure of a series of shearing elements rather than simultaneous failure of such shearing elements, thereby introducing uncertainty in the magnitude of the lifting force that results in release of the packer mill from the remains of the packer, and possible premature release.
It is therefore desirable to provide a means for releasing a packer mill from the remains of a packer with a high degree of reliability and without inherently producing loose parts that may cause further difficulty in the well bore. The release mechanism should not cause damage to the packer mill that prevents prompt reuse of the mill.